Conventionally, an image forming apparatus such as an electrophotographic printing machine adopts one of two developing system: (1) a normal developing system for carrying out development by adhering toner to an unexposed portion of a photoreceptor and (2) a reversal developing system for carrying out development by adhering toner to an exposed portion of a photoreceptor. In the case where development is carried out by using a photoreceptor to which a charge having a negative polarity is applied, for example, in the normal developing system, development is carried out by using toner having a positive polarity, and on the contrary, in the reversal developing system, development is carried out by using toner having a negative polarity which is same as a charge polarity of the photoreceptor.
As to an image forming method which adopts the reversal developing system, Japanese Examined Patent Publication NO. 2-4900/1990 (Tokukohei 2-4900) discloses a "picture image forming method" and Japanese Examined Patent Publication 3-64864/1991 (Tokukohei 3-64864) discloses an "image forming method".
In the "picture image forming method" disclosed in Japanese Examined Patent Publication 2-4900/1990 (Tokukohei 2-4900), a photoreceptor to which a light transmitting electrically conductive layer, a photoconductive layer and an insulating layer are provided in this order is used, and electrically conductive and magnetic toner carried by magnetic force is supplied to a surface of the insulating layer in the photoreceptor. Next, while a voltage is being applied between the toner and the light transmitting electrically conductive layer, the photoconductive layer is exposed so that a pair of charges having an opposite polarity is formed between the photoconductive layer and the toner between which the insulating layer is put in an exposed portion. As a result, the toner is electrostatically absorbed on a surface of the photoreceptor by defeating magnetic force for carrying the toner. Meanwhile, toner absorbed on the unexposed portion is removed from the surface of the insulating layer by the magnetic force after the exposure. In such a way, a desired image is formed.
In addition, in the "image forming method" disclosed in Japanese Examined Patent Publication 3-64864/1991 (Tokukohei 3-64864), toner having conductivity and magnetism is allowed to contact with a photoreceptor, and a voltage is applied to the photoreceptor. Next, a light having image information is irradiated from a conductive layer side of the photoreceptor, and the toner is allowed to electrostatically adhere to an exposed portion of the photoreceptor so that a toner image is formed. In such a way, a desired image is formed.
In addition, as an information processing apparatus such as a computer has been in common use in recent years, more printers which adopt an electrophotographic printing method have been used as printing means for information. Such a printer, which adopts the electrophotographic printing method, generally adopts the reversal developing system as a developing method. Meanwhile, a copying machine which adopts an electrophotographic printing system generally adopts the normal developing system as a developing system.
Incidentally, in general, since a developing system of a copying function is different from that of the printer function, the polarity of toner to be used for development is also different. For this reason, it is hard to form an image by means of both the copying function and the printer function through a series of image forming processes using one photoreceptor. For this reason, in order to superimpose an image on a copy image using a printer, a document is copied by a copying machine, and it is necessary to carry out a desired printing on copied paper by a printer. As a result, the machine has a problem of operability, so there arises a problem that the time it takes to form an image by superimposing an image on a copy image by a printer becomes long.
In addition, in order to perform the copying function and the printer function in one machine, a machine, which adopts the normal developing system for printing an unexposed portion as the printer function is suggested. However, in this case, since a ratio of a light emitting time of writing and exposing means to an operating time of the printer function exceeds 90%, there arises problems such as overheat of the writing and exposing means, a reduction in a service life. Moreover, there is a possibility that an unexposed portion to be printed is blotted with an exposed portion, thereby arising a problem that an excellent image cannot be formed.
Furthermore, since in the image forming method which adopts the reversal developing system, an exposed portion is developed, it is necessary that a photoreceptor is charged with a high voltage. For this reason, a charging unit which generates a high voltage is required. Therefore, there arises a problem of increased size of an apparatus.